1. Field of Invention
The invention relates to apparatus and processes for growing oxides on silicon wafers, silicon-coated wafers or other semiconductors, as used in the semiconductor industry.
2. Description of Prior Art
Oxygen or steam is commonly used for growing oxide on silicon wafers. As the oxide grows thicker, it forms more slowly. In semiconductor manufacturing there has been a developing need for thicker oxide coatings, which, under available techniques having relatively low growth rates, require very long periods of processing time. In an effort to speed up the growth rate, the temperature in the reaction chamber has been increased in the range of about 1,000.degree.-1,200.degree. C. However, high temperature may have a disturbing effect on previously diffused work, and processors would, accordingly, like to hold the temperature down to a range of about 700.degree.-950.degree. C., which, in turn, prolongs the oxidation process.
Another developing need in semiconductor manufacturing is the use of larger diameter silicon wafers so as to obtain a higher yield of usable semiconductor chips, which generally increases as the square of the diameter of the wafer. The wafers are, however, very thin, typically 10-15 mils., and as the size increases, so do mechanical gradients, which are exacerbated at high temperatures. Accordingly, oxidation of the wafers at lower temperatures, say 700.degree.-950.degree. C., will permit the handling of larger diameter wafers, with their significantly increased chip yield.
It is known that the rate of growth of oxide on a silicon wafer in a proper environment is a function of time, temperature and pressure, see article entitled "Oxidation of Silicon by High-Pressure Steam," by Ligenza, Bell Telephone Laboratories, pages 73-76, Journal of the Electrochemical Society, February, 1962, and Abstract No. 53 entitled "High Pressure Steam Apparatus for the Accelerated Oxidation of Silicon," by Panousis and Schneider, Bell Telephone Laboratories, Spring Meeting of the Electrochemical Society in Chicago, May 13-18, 1973.
In the first of the noted publications, experiments were conducted in sealed bombs. The silicon sample and a measured amount of water were sealed in the bomb and the latter heated to a range of about 777.degree.-1123.degree. K. The measured water quantity/temperature was calculated to produce a pressurized steam atmosphere within the bomb of at variously between 25-400 atms. for times ranging from 1/2 hour to 8 hours.
The structure described in Abstract No. 53 represented some advance in that the steam pressure within the reaction chamber was established by a pressure regulator, rather than the more imprecise water calculation. The structure included a boiler for generating steam, which was fed through a pressure regulator into a stainless steel pressure vessel containing the silicon slices to be oxidized, and a surrounding alumina sleeve having heating coils therearound for raising the temperature within the chamber into the range of 900.degree.-1,200.degree. C. In this structure the entire interior of the chamber was subjected to the oxidizing effect of the steam, and no attempt was made to isolate the wafers for processing in a "clean room" condition. Also, water, however prepared, filtered, distilled, etc., is still likely to have chemical particulate contaminants which cannot be tolerated in semiconductor manufacturing.
For additional prior art, see article entitled "Selective Oxidation of Silicon in Low-Temperature High-Pressure Steam," authored by Powell, Ligenza and Schneider and published in IEEE Transactions on Electron Devices, Vol. ED-21, No. 10, October, 1974. High-pressure systems, while demonstrated in the laboratory, have not been available in commercial structures.